1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus having a recording head and a head tank, the recording head discharging liquid droplets, the head tank supplying a fluid to the recording head.
2. Description of the Related Art
As an image forming apparatus such as a printer, a facsimile machine, a copier, a plotter, a multi function peripheral thereof and the like, there has been known an inkjet recording apparatus and the like employing a liquid discharging recording method using a recording head discharging ink droplets or the like.
In the image forming apparatus employing the liquid discharging recording method, an image is formed by discharging ink droplets from a recording head onto a fed sheet. Herein, the term “forming” is a synonym of the terms recording, typing, imaging, and printing.
Further, herein, the term “sheet” is not limited to paper, and refers to any appropriate medium (e.g., the OHP) to which ink droplets, other liquid and the like may be adhered. The image forming apparatus employing the liquid discharging recording method may be classified into two types: a serial-type image forming apparatus and a line-type image forming apparatus.
In the serial-type image forming apparatus, an image is formed by discharging ink droplets from the recording head while the recording head moves in the main scanning direction. On the other hand, in the line-type image forming apparatus, an image is formed by discharging ink droplets from the line-type recording head while the recording head does not change its position.
Further, in an embodiment of the present invention, the term “image forming apparatus” employing the liquid discharging recording method refers to an apparatus forming an image by discharging a liquid onto a medium including paper, thread, fiber, textile, leather, metal, plastic, glass, wood, ceramics and the like.
Further, the term “image forming” refers to not only forming a meaningful image such as characters, figures, and the like on a medium but also forming a meaningless image such as a pattern and the like on a medium (including simply discharging droplets onto a medium).
Further, the term “ink” is collectively used and herein refers to not only any material called “ink” but also any liquid for forming an image which may be called recording liquid, fixing processing liquid, liquid, a DNA sample, a patterning material, a resin and the like.
Further, the “image” is not limited to a planar image. For example, the “image” includes an image formed on a material that is three-dimensionally formed and an image three-dimensionally formed made of three-dimensional figures.
Among such image forming apparatuses there is a known image forming apparatus employing an ink supply method in which ink is supplied from a main tank to a head tank, the main tank (a.k.a. an ink cartridge) being detachably mounted on a main body of the apparatus, the head tank (also called a sub tank or a buffer tank) supplying ink to the recording head.
When the ink cartridge is in an ink-end condition (an end state) (i.e., an ink-empty condition (an empty state)), including an ink-near-end condition (i.e., an ink-near-end condition), exchange of the ink cartridge may be suggested. However, when there is no stock of the ink cartridge, the ink cartridge may not be exchanged immediately. As a result, printing may have to be interrupted.
To overcome the inconvenience, there is a know technique in which emergency printing using ink remaining in the head tank is performed to continue printing (see, Japanese Laid-open Patent Publication No. 2011-183729).
To that end, in a case where ink is supplied from the main tank to the head tank, when the pressure in the head tank is less than a predetermined pressure, an air opening mechanism of the head tank is opened to increase the pressure in the head tank. Then, the liquid is pumped from the head tank to the main tank by a reversible-type fluid supply unit to control reducing the negative pressure in the head tank.